fix(#284): remove dead code across API, models, WS client, and docs (#299)

Remove six dead-code findings from the automated review:
- CODE-01: updateProfileDescription endpoint + UpdateProfileDescriptionRequest DTO (uncalled)
- CODE-02: getConfig() endpoint (app uses getRawConfig())
- CODE-03: TogglePluginRequest DTO (unreferenced)
- CODE-05: mark isConnected with @VisibleForTesting (test-only usage)
- CODE-06: fix KDoc createdAt -> timestamp to match actual field name

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/data/local/ChatMessageEntity.kt

@@ -9,7 +9,7 @@ import androidx.room.PrimaryKey
  * UI model but is stored independently so it survives process death.
  *
  * Messages are scoped by [sessionId] so switching sessions loads the right
- * thread. The [createdAt] field preserves original ordering even if Room
+ * thread. The [timestamp] field preserves original ordering even if Room
  * reorders internally.
  */
 @Entity(tableName = "chat_messages")